High resoulution potentiometer



Aug. 27, 1968 W. l. ELLIOTT ETAL HIGH RESOLUTION POTENTIOMETER vINVENTORS United States TIC@ 3,399,368 HIGH RESOLUTION POTENTIOMETERWilliam I. Elliott, 5172 Dumont Place, Woodland Hills, Calif. 91364, andJack L. Randall, 27 Malibu Colony, Malibu, Calif. 90265 Filed Jan. 24,1966, Ser. No. 522,736

Claims. (Cl. SSS-177) This invention relates to apparatus using variableresistance devices, and more particularly to a unique construction forpotentiometers and the like with which to obtain infinite resolutionwith a wire-wound resistor.

It has long been known to apply a voltage across the terminals of aresistor element formed of many turnsuof resistance wire, and to moveanv electrical wiper contact, or pick-off, from turn to turn to changethe voltage between either terminal andthe wiper. The greater the numberof turns, the smaller is the increment of change in voltage from turn toturn. Thus, a resistor may be formed of 2,500 turns per inch of wirewound on a mandrel, in which case movement of the wiper along themandrel between adjacent turns effects a change of 1/2500,` or

0.0004 of the total voltage across either 4terminal of the resistor andthe wiper. B ut this is the smallest voltage change that can beeffected, or detected, 'with' this arrangement. 4

However small the wire, and howeverlarge the number of turns, it has notheretofore been possible with such wire-wound resistors to obtain inniteresolution, i. e., to l effect as small a voltage change as desired. Inmany' cornplex electronic systems, it is often desirable to change avoltage applied to a circuit by 'as little as a` lrnillio'nth of theapplied voltage.

Variable resistors with which infinite resolution can be' obtained havebeen made with powderedv resistive and ceramic materials mixed togetherand sintered to form a monolithic element. In some, the proportions ofingredients in the mixture are the same throughout the length of theelement, but the thickness of the element varies from end to end. Inothers, the element is of uniform thickness along its length, but thequantity of resistive material varies gradually from end to end. Ineithercase, the wiper can be moved to any ldesired position on theelement to effect any desired voltage change.

However, there are certain disadvantages in thel use of such resistorelements. Dueto theirparticulate structure, they manifest relativelyhigh electrical noise levels and temperature coeicients of resistance,which are greatly improved by resistors wound with wire of solid metal.

It is an object of our invention to provide wire-wound resistorapparatus with which to achieve infinite resolution.

It is another object of our invention to provide a unique Wire-woundresistor and wiper therefor which function both as electrical elementsand as cooperative threaded means to make the wiper follow the turns ofthe resistor. y y

A further object of our invention is to provide a potentiometer or likestructure having a minimum number of component parts of simple design`and rugged construction. A

The ,above and other objects and advantages of our invention will becomeapparent from the following description taken in conjunction with theaccompanying drawing of an illustrative embodiment thereof, in which:

:FIGURE 1 is a longitudinal sectional View of a potentiometer of ourinvention, wherein the windings of the resistor element constitutethreads for mating engagement with threads of an element which functionsboth as the wiper and a traveling nut; l

FIGURE 2 is a sectional View taken along the lines 2 2 of FIGURE l,showing more clearly the arrangement of the wiper-nut element;

FIGURE 3 is a fragmentary sectional view taken along the lines 3--3 ofFIGURE l, to aid in explaining the movement of the Wiper-nut along theresistor element;

FIGURE 4 is an enlarged fragmentary sectional view of a portion of theresistor element encircled by the wiper nut, to aid in explaining themethod of our invention in forming the cooperative threads; and

FIGURE 5 is an enlarged fragmentary sectional view of the wiper-nut atone end of the resistor element at which further linear movement thereofis prevented, to aid in explaining the operations of the mating parts toprevent stripping of the threads thereof.

Referring to FIGURES 1 and 2, la potentiometer 10 in Aaccordance withour invention employs ya housing 11 having a central opening 12 therein,in which a mandrel 13 is adapted to rotate. The mandrel 13 carries aresistance winding 14 on its outer surface, the turns of which formmating threads with a plastic nut 15, which is formed as a broken ring,and which is supported in a spring clip 16 which at one end forms aconductive wiper contact 17 engaging the winding 14. The Ispring clip 16is provided with a projection 18 that rides in the groove or keyway 19formed in an elongated channel-shaped member 20. The channel 20 is ofconductive material, and is electrically connected at one end to anexternal pin 21, and the ends of the winding 14 are connected toconductive slip rings 22, 23 which rotate with the mandrel 13, andwhich, through respective wiper contacts 24, 25, are electricallyconnected to external pins 26, 27.

. In this manner, rotation of the mandrel 13, and hence the resistorwinding 14, effects longitudinal movement of the nut 15, the directionof travel of the nut depending upon the direction of rotation of themandrel 13. In this manner, the wiper 17 is caused to follow the turnsof the winding 14 as it moves from one to the other thereof.Accordingly, any desired adjustment in the position of the nut 15 can bemade to effect any 4desired change in voltage at the pin 21.

For example, assume the winding 24 to -be one inch long, and to comprise2500 turns of wire, the ends of which are connected, via terminal pins26, 27, to a source of voltage (not shown). One revolution of themandrel 13 causes the wiper 17 to traverse the length of wire whichaccounts for 1/500 or 0.0004, of the total applied voltage. However,inasmuch as the wiper engages such turn throughout `such a revolution, afraction of the revolution effects a corresponding fraction-al change inthe portion of the voltage represented by a complete turn. Thus, movingthe mandrel 113 through an Iangle of one degree effects a change of$6550 of the 0.0004 fraction of the applied voltage, or approximatelyone-millionth of such applied voltage. Movement of the mandrel through afraction of a degree effects a correspondingly smaller fractional changeof the applied voltage.

It will be appreciated that it is impossible by conventional methods tomake :a threaded element with threads of as slight a pitch as aboveindicated. Even by engraving methods, it would be next to impossible toform threads of the order of a 0.0004 inch pitch with accuracy.Referring to FIGURE 4, in our invention we utilize the resistor winding14 for forming the internal threads of the nut 15. To this end, we firstwind the resistance wire on the mandrel 13, it being understood thatsuch wire, however small it is in diameter, is coated with insulation30. The turns of the winding 14 are -attached to the mandrel 13, as by acoating 31y of plastic material, such as an epoxy resin, which forms abond between the mandrel 13 and the confronting portions of theinsulation 30 around the wire.

In this latter connection, the winding may be attached to the mandrel inany desired manner. For example, the thin coating of the plasticmaterial m-ay be applied to the mandrel lbefore winding the wirethereon. The wire is wound on the mandrel so that the insulationcoatings on adjacent turns are in abutment. If the plastic coating 31 ishardened at the time the winding is wound thereon, the assembly isplaced in an oven and heated to cause the plastic material to flow andlill the helical groove that is defined by the diverging portions of theinsulation 30 and the outer surface of the mandrel. If the coating 31 iswet when the Wire is wound on the mandrel, the plastic material fillssuch groove, whereupon subsequent curing of the plastic material causesthe winding to be firmly Ibonded to the mandrel.

Further, the winding may be wound on a dry, i.e., uncoated, mandrel. Theends of the mandrel are plugged, and it is immersed in a container ofliquid materi-al to be used for bonding. The container is then placed ina vacuum chamber, and a vacuum is drawn to cause the liquid to be drawnbetween the mandrel and the winding, and to :fill all intersticestherebetween. The assembly is then removed from the vacuum and thebonding material is allowed to harden.

lAfter the winding 14 is bonded to the mandrel 13, the Aouter portion ofthe insulation 30 around each turn of wire is removed. Removal of theinsulation from the wire can be effected by any known technique, as -bysoda bufling, wherein an abrasive of finely divided particles of sodaare applied to the external portions of the insulation by a sandblasting technique. Alternatively, the insulation may be removed bysimply rubbing it off with a soft eraser.

For purposes of ya precision potentiometer, the mandrel 13 is ofsufficiently large diameter as to avoid undue stress in the wire by-virtue of wrapping it around a cylinder. As will be appreciated,winding any wire on a mandrel that is too small in diameter requiresthat the wire follow such a short radius as to set up stresses therein,which would be greatest where the wire engages the mandrel. The effectsof such stresses become pronounced over wide temperature variations, andresult in marked changes in the temperature coefficient and theresistance of the wire in different temperature ranges. Where suchchanges cannot be tolerated, we employ a mandrel that is of sufficientoutside diameter as to substantially eliminate this possifbility. Forexample, in one application using wire of 0.0004 inch diameter, weemploy a mandrel having an outside diameter of approximately 0.25 inch.

Also, where the apparatus is subjected to a wide range of temperatures,the material 'used for the wire preferably has a temperature coecientwhich is as near-ly like that of the mandrel as possible. The mandrelmay be made of brass, which has an extremely low temperaturecoeflicient, in which case the material used for the wire of the winding14 should be made of alloys of metal having a comparable coefficient ofexpansion and contraction.

As previously mentioned, the nut 15 is formed of plastic material. Ifthe material of the nut 15 is one that is capable of being ldeformed bythe application of pressure alone, e.g., Teflon, the spring clip 16surrounding the material is utilized for this purpose. Since theelements forming the wiper and the nut are split rings, the constantinward pressure of the spring clip 16 results in grooves 32 (FIG. 4)which conform to the exposed portions of the wire, and ribs 33 formed ofthe portion of the material that is forced inwardly toward the portionsof insulation 30 between adjacent turns of the wire.

As will be seen, this arrangement results in the formation of internalthreads for the nut 15 which matingly engage the turns of the Winding,so that rotation of the mandrel 13 effects linear travel of the nut 15in a manner conventional for linear actuators.

Where the nut 15 is to be formed of a plastic material which isextremely hard and has long-wearing qualities, c g., a nylon moldingcompound, the force exerted by the clipy 16 is insufficient to deformthe internal surface of the nut to provide the desired internal threadstherefor. In such case, the split ring for the nut is molded, and theninserted in the clip 16 and bonded thereto with a suitable plasticmaterial. Such bonding may be facilitated by roughening the outersurface of the molded ring, as by etching, before bonding it tothe'spring clip 16. The clip 16 and the plastic ring therein are snappedonto the exterior of the winding 14. We then heat the internal surfaceof the plastic ring to a sufficient temperature to cause the material toflow, whereupon the pressure exerted by the spring clip 16 forces thematerial into the grooves between adjacent turns of the wire, thereby toform the internal threads for the nut as previously described inconnection with FIGURE 4.

The application of heat for molding the threads in the inner surface ofsuch a hard plastic element may be effected in a variety of ways. Onetechnique is to apply a heating current through the wires of the winding14 of sufficient magnitude to heat the internal surface of the element.Due to the continuing inward pressure exerted by the clip element 16,the softening of the inner surface of the element forces it inwardly toform the grooves and ribs 32, 33 as shown in FIGURE 4.

Another technique for carrying out this heating step is to place theentire assembly in an oven and heat to a sufiicient temperature toeffect the desired molding of the threads.

Molding of the threads on the nut 15 may be accomplished on a volumebasis. To this end, we employ a master mandrel wound with a resistancewinding as described herein, and position a plurality of assembledplastic rings in spring clips around the winding. The heating steppreviously described thus forms internal threads on all of the plasticnut elements simultaneously. After such molding, each of the wiper-nutassemblies is removed from the master unit and pla-ced on a respectivewire-wound mandrel, on which wire of the same diameter has been wound.

For a fuller understanding of the operation of the potentiometer of ourinvention, reference is made again to FIGURES 1-3. As shown, the housing11 is formed as an elongated cylinder having an end cap 40 bonded inplace as by a suitable plastic adhesive coated on the confrontingsurfaces of the cylinder and the end cap. The housing parts may beformed of any desired material, e.g., metal or plastic.

As shown at the right-hand end of the housing in FIG- URE 1, the end cap40 has an inner cylindrical projection 41 which is surrounded by ametallic sleeve 42 forming a sleeve bearing. Surrounding the bearing 42is a metallic sleeve 43, which is press-fit into a cylinder 44 whichextends into the end of the mandrel 13. The confronting surfaces of themandrel 13 and the cylinder 44 are bonded together, as with a suitableepoxy cement. The flange 45 of the cylinder 44 abuts the ends of themandrel 13, and extends radially a substantial distance beyond thewinding 14. As shown, the slip ring 23 is mounted on the periphery ofthe flange 45, to which it is bonded. The adjacent end of the winding 14is suitably connected to the slip ring 23, as by extending it through agroove in the radial face o'f the flange 45,"a`nd soldering or weldingit at 46 to the inner Surface of the slip ring 23.

At the opposite end of the mandrel 13, the slip ring 22 is mounted onthe periphery of the flange 48 of a 'cylinder 49 that extendsinto and isbonded to the interior surface v of the mandrel 13, in lthe manner ofthe cylinder 44. The outer diameter of the fiarige 48 is the same asthat of the flange 45, and the slip ring 22 is the samey size as theslip ring 23. As in the case ofthe terminal of the winding connected tothe slip ring 23, the end of the winding adjacent the fiange 48 issimilarly connected at 50 to the slip ring 22.

The cylinder 49 is press-fit on the inner en-d of a stub shaft 51 thatextends to therexterioru:` the housing ,11. As shown, the stub shaft 51is rotatably mounted in a bushing 52 which is secured inthe left end ofthe housf ing. The outer end of the stub shaft 51 may be provided with ascrewdriver slot '53 through which to rotate the mandrel manually, orthe stub shaft may be adapted as at 54 for operation by a motor 55.Thus, it will be seen that the shaft 51, the mandrel 13 with the winding14 thereon, and the slip rings 22, 23 are arranged for unitary rotation.

As seen in FIGURES 1 and 2, the wiper contact 25 extends into a groove56, Where it is soldered or welded to one end of a wire 57 extending thelength of the groove from the inner end of the terminal pin 27. Asshown, the Wire 57 is laid in the bottom of the groove 56, and is pottedin place, by filling the groove with a suitable plastic material,indicated at 58. At the right end of the housing, the wiper 26 extendsinto a similar groove 59, where it is soldered or welded to a wire 60extending from the inner end of the terminal pin 26, as in the case ofthe wire 57 and terminal pin 27. Also, as in the case of the pottingmaterial 58 in the groove 56, the groove 59 is filled with pottingmaterial 61.

To aid in maintaining the desired electrical contact between the winding14 and the terminal pin 21, a short metallic leaf spring 62 is solderedor welded to the projection 18. As shown, the leaf spring 62 is solderedto the radially extending portion of the projection 18, and is bent overthe upper end of the projection. The spring 62 is self-biased outwardlyfrom the end of the projection 18, so as to be maintained in continualengagement with the inner surface of the web portion of the channel 20.

The wiper-nut construction in our invention offers the unique advantagethat When the nut engages either of the flanges 45, 48 at the ends ofthe mandrel, continued rotation of the mandrel in the same directionwill not result in damage to the cooperating threads of the winding 14Iand the nut 15. In other words, our invention dispenses with therequirement in prior art linear actuators for mechanical slip clutchesand/ or electromagnetic clutches operated by switches disposed in thepaths of the traveling elements.

Referring to FIGURE 5 along with FIGURE l, when the nut 15 is brought tobear against the flange 45, the split ring configuration of the clip 16and the nut 15 permit the threads constituted of the winding 14 to camthe nut 15 and clip 16 outwardly during those portions of the revolutionin which the threads would, with a nut formed of a solid ring, result instripping of the threads or alternately, causing the interfering threadsto bind and thereby place an overload on the motor 55. By this means, ifthe mandrel 13 continues to turn in the same direction, the nut 15simply opens and closes as long as the mandrel is turned, but withoutdamage to any of the parts of the potentiometer or the actuating meanstherefor. As soon as the direction of rotation of the mandrel isreversed, the threads of the nut immediately resume mating engagementwith the turns of the winding, to cause travel of the nut in theopposite direction. I

To facilitate the above-described operation of the wipernut arrangement,and to avoid any short-circuiting prob- '6 lems, 'we make the axiallengthof the nut and clip 16 greater than the axial dimensions of theprojection 18 and the portion of the spring wiper 62 that is welded orsoldered thereto. Additionally, the outer diameter-'of the metal clip 16is smaller than the inner diameter of the slip-rings 22, 23. Thus, whenthe nut reaches the end of its travel at either end of the mandrel, noneof the metal parts associated with the clip 16 can engage a slip ring.

It'will be noted that wehave illustrated the nutas spanning severalturns of the winding 14. Fora nut of given axial dimension, the numberof turns of the winding spanned thereby, i.e., the -number of threadsformed in the nut, will Idepend uponthe size of the wire. For wire ofthe order of 0.0004-in. diameter, a nut of 0.080-in. axial length spansof the order 'of 200turns of the winding 14. For awinding 14'wound` withwire of 0.016-in. diameter, a nut of such'axial dimension spans five (5turns. Correspondingly, of course, the wiper portion 17 of the clip 16spans a number of turns of the winding, the number of which also dependsupon the size of the wire used. We find that this arrangement isbeneficial, in that it insures mechanical stability of the nut, i.e., bypreventing tilting thereof as it is moved along the mand-rel.

From the foregoing, it will be apparent that various modifications canbe made in the method and apparatus as illustrated and described herein,without departing from the spirit and scope of our invention.Accordingly, we do not intend that our invention be limited, except asby the appended claims.

We claim:

1. Variable resistance apparatus comprising:

a member having a supporting surface;

Ia resistance wire wound in closely spaced turns of equal diameterconfronting said surface;

insulating and bonding material located between and bonding togethersaid surface and the portions of the turns of wire confronting saidsurface, portions of said material also extending between and bondingadjacent t-urns of the wire together;

a plastic element located adjacent the wire, said plastic element andsaid material being separated by the wire, the portion of said plasticelement nearest said wire being an arcuate surface having groovesconforming to and mating with the portions of the wire confrontedthereby, and ribs between the grooves which fit between adjacent turns;

means biasing said plastic element against the portions of the wireconfronted thereby, the turns of wire and the ribs of said plasticelement being in meshing relation and coacting so that relative rotationbetween said member land `said plastic element effects relative axialmovement therebetween; and

=a contact element carried by said plastic ele-ment and engaging thewire on at least one turn thereof.

2. Apparatus as defined in claim 1, wherein:

said member is a cylindrical mandrel,

said wire being wound on the outer surface of said mandrel,

said plastic element being a split plastic ring, and

said biasing means being an inwardly biasing split ring clip ofconductive metal surrounding said plastic ring and coextensivetherewith, said plastic ring being attached to said clip, and

said contact element being carried by said clip and extending into tbespace between the ends of said clip to engage the wire.

3. Apparatus as defined in claim 2, wherein said insulating and bondingmaterial includes insulation surrounding the portions of the turnsadjacent said mandrel and extending between and separating the wire ofadjacent turns; and a plastic bonding material securing said insulationto the outer surface of said mandrel.

4. Apparatus as defined in claim 3, further including:

la housing of nonconductive material having an opening therein;

y"a channel of conductive material secured in said hos-v ing and spacedfrom said spring clip; and

"a conductive 'radial projection on said clip extending into saidchannel, wherebjfrotation of said mandrel l imparts longitudinalmovement to said clip 'and said plastic ring, l

5. App4 a'tusv .as defined in claim 4, including Ian externaltefrniinal'lextending at one end into said housing;

a conductor connectingsai'd channel to said terminal;

nonconductive cylindrical memberssupporting the ends' o'f`rsaid. mandreland being adapted for rotation vvin said housing, ,saidrmer'nbers havingradial ang'es adjacent the ends of saidv mandrel, the outer surface ofeach flange being of greater diameter than the outer diameter of 'saidclip;

respective slip ring surrounding and being secured lto the nuter surfaceof each flange, the slip rings*` being conductively connected to theadjacent ends( v I of said wire;

a pair of wiper contacts supportedvin said :housing atl opposite ends ofsaid mandrel, each wiper` contactf. i engagingarespectiveone of saidslip rings;` y .'apair of external terminals supported on said housing;p and f conductorsconnecting the wiper contacts to the re;A 10i igspectiveterminals of said pair.

i y References `Cite/il` UNITED STATES PATENTS 15' 3,069,646 V12p/,P1962Hardison et a1. .v 338--15111l ROBERT K. SV CHAEFER, Primary Examiner.

H. HQHAUSEN, Assistant Examiner.

